This invention relates generally to optical beam steering and, more particularly, to a subaperture-addressed optical beam steerer providing high performance optical beam steering of large aperture beams, and methods for providing an enhanced multiplicity of steering angles in such a device.
A static deflector for deflecting a polarized infrared beam is suggested by U.S. Pat. No. 4,639,091, issued Jan. 27, 1987, to J.-P. Huignard et al. The Huignard et al. deflector comprises a layered square plate having as a front layer a window on which stripe electrodes are disposed. Both the window and the stripe electrodes are transparent to an incident infrared beam. A middle layer of the deflector comprises an electro-optical liquid crystal layer. The bottom layer comprises a substrate having a common electrode adjacent the liquid crystal layer. The common electrode is preferably reflective at the beam wavelength, illustratively it is a gold film; alternatively, for a deflector operating by transmission, a transparent rear plate may be used.
Huignard et al. suggest a periodic staircase waveform comprising N voltage steps which are applied to the stripe electrodes, thereby creating local variations of the refractive index in the liquid crystal layer in such a manner as to form a diffraction grating of adjustable period.
Practical applications of the striped-electrode, liquid crystal cell optical beam deflector concept are disclosed in U.S. Pat. No. 4,964,701, "Deflector for an Optical Beam," issued Oct. 23, 1990, to Terry A. Dorschner et al., which patent is incorporated herein by reference, and U.S. Pat. No. 5,018,835, "Deflector for an Optical Beam Using Refractive Means," issued May 23, 1991, Terry A. Dorschner. These, as well as other applications of optical beam steering, underscore the need for rapid, large-angle pointing and scanning of optical beams, in particular, large diameter, diffraction limited carbon dioxide (CO.sub.2) laser radar beams. In short, there exists a pressing need for an optical version of the versatile phased array antennas now widely used for microwave radar systems.
An optical phased array "antenna" for electronic steering of optical beams is difficult to realize in practice because of the very large number of phase shifters and the corresponding very high density of electrical connections required for operation of an optical array. High performance, large-angle beam steering requires that the individual phase shifters of the array have spacings less than the wavelength of the light to be steered. Spacings of one-half to one wavelength are usually chosen for microwave phased array antennas, and it is anticipated that comparable spacings will be used in optical systems.
Fabrication of liquid crystal optical phase shifters of this dimension is quite feasible using semiconductor photolithography. At the present time, devices with electrode widths of less than two microns are being readily fabricated. Additionally, sub-micron spacings are feasible with state-of-the-art lithography means. However, connecting each of the phase shifters of a large array to independent voltage supplies appears to be monumental task.
Considering the more-or-less optimal case of one-half wave spacings, if all of the phase shifters of a linear, one-dimensional array were to be independently addressable, the edge connection density would be 2000 per centimeter (cm) of aperture at ten microns wavelength, and 20,000 per cm at one micron wavelength. Since apertures up to one meter are desired, the number of electrical connections required for a conventionally-operated phased array architecture may be one million, or even larger for visible wavelengths. A second one-dimensional unit to cover a second dimension of steering would require an equal number of connections. Numbers of off-chip interconnects of this magnitude are considered to be vastly excessive, especially considering that current practice is to use no more than a few hundred off-chip connects in conventional semiconductor technology.
Optical phased array systems are also known in which the phase shifters and spacings are larger than a wavelength, with consequential performance degradation. The resultant reduction of phase shifters obviously reduces the required number of electrode connections. Nevertheless, this approach is considered unacceptable for many applications since spacings larger than one wavelength generally give rise to multiple output beams for a single input beam. Where the application of the present invention is in a laser radar system, it is generally essential that there be only one beam. The presence of multiple beams may be tolerable for some transmitting functions; the power into the intended beam is merely reduced, albeit by a large factor. However, in the receive mode, simultaneous sensitivity to energy from multiple directions can give rise to an unacceptable ambiguity in the target direction.